error[E0503]: cannot use `y.1` because it was mutably borrowed
  --> $DIR/match-cfg-fake-edges2.rs:8:5
   |
LL |     let r = &mut y.1;
   |             -------- `y.1` is borrowed here
...
LL |     match y {
   |     ^^^^^^^ use of borrowed `y.1`
...
LL |         (true, _) => drop(r),
   |                           - borrow later used here

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0503`.
